1. Field of the Invention
This invention pertains generally to the field of superconducting inductive devices and more particularly to superconducting inductive devices and energy storage devices having magnetic shields.
2. Description of the Prior Art
There are many instances where it is desirable to be able to store energy for various periods of time, and return it to a power distribution system as needed. One example is the electric power industry, wherein the load on the power systems is not constant but fluctuates markedly from hour to hour. In the course of a typical day, the peak load during the day-time may be on the order of twice the maximum night-time load. At present, pumped water storage is the only means being employed by the power industry to achieve even a small degree of load leveling. It may also be desirable and even necessary to have energy storage devices capable of augmenting the commercial supply of power to a load which requires large amounts of energy for short periods of time, wherein the average power requirements are moderate but peak power requirements are very high.
One type of energy storage device receiving increased attention is a superconducting magnet or inductor, which is capable of maintaining a large current for very long periods of time with negligible energy loss in the inductor itself. However, one of the problems encountered in using superconducting inductors for energy storage purposes or for transformer, motor or generator windings is that for high energy exchange rates, the rapid changes in fields and currents cause energy losses in the superconducting winding. These losses are in fact enhanced for a typical stabilized superconducting magnet in which there are superconducting filaments buried in a matrix of normal metal, such as copper or aluminum. In a typical case the losses are possibly 100 times greater in the composite conductor made up of copper plus niobium-titanium filaments as compared to an equivalent size conductor of copper alone. These losses are primarily due to so-called coupling losses, and the eddy currents developed in the copper stabilizer. Excessive eddy currents can generate a prohibitive amount of heat and localized high intensity magnetic fields, which may result in loss of superconductivity in small portions of the superconductor with the result that still more heating and loss of superconductivity occurs. To avoid these losses it is thus desirable to minimize the variation in the current and the magnetic fields in the superconductor.